dt-bindings: pwm: renesas: tpu: Fix "compatible" prop description

The "compatible" property description contradicts even the example given:
it only says that there must be a single value while the example has the
fallback value too -- which makes much more sense. Moreover, the generic
property value is misdocumented as being R-Car (and RZ/G1) specific...

Fixes: 382457e5 ("pwm: renesas-tpu: Add DT support")
Fixes: 3ba111a0 ("dt-bindings: pwm: renesas-tpu: Document r8a774[35] support")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>